Heme oxygenase-1 (HO-1) plays a role in the resistance to apoptosis of several types of cells, but its role in the development of thyroid cancer is unknown. In this study, we investigated the regulation of HO-1 in human papillary thyroid carcinoma cells (KAT5). The results show that HO-1 is significantly induced by hemin and cadmium. In addition to inducing HO-1, hemin and cadmium also cause a rise in the levels of p21, a cyclin-dependent kinase inhibitor. Cells with increased levels of HO-1 and p21 were more resistant to apoptotic stimuli than cells with normal levels. The cells resistant to apoptosis also displayed an increased arrest at the G0/G1 phase of the cell-cycle. The induced levels of HO-1 and p21 were significantly reduced by p38 mitogen-activated protein kinase (p38 MAPK) and extracellular-regulated kinase (ERK) inhibitors. More importantly, KAT5 cells regained their sensitivity to apoptotic stimuli after they were treated with these kinase inhibitors, indicating that p38 MAPK and ERK are required for the resistance to apoptosis conferred by HO-1. Furthermore, we demonstrated that increased levels of HO-1 and p21 expression are associated with an increase in the activity of NF-kappaB and that inhibiting NF-kappaB leads to a block in the induction of HO-1 and p21. In summary, this study reveals that an increase in the level of HO-1 markedly reduces the sensitivity of papillary thyroid carcinoma cells to apoptotic stimuli. The HO-1 pathway of apoptosis resistance is associated with an increase in the levels of p21, involves a p38 MAPK and ERK-mediated mechanism and can be suppressed by inhibiting NF-kappaB. © 2004 Wiley-Liss, Inc.